1. Field of the Invention
The present invention refers to a method as well as to an apparatus for cold-roll forming of annular workpieces into a desired profile, particularly for manufacturing vehicle rims consisting of aluminium.
2. Prior Art
Devices are known for the cold-roll forming an annular workpieces, particularly for the profiling of vehicle rims, which comprise two tools in the shape of solids of rotation acting on a workpiece from opposite sides, the mutual distance of the tools being variable. In order to shape e.g. a vehicle rim, it is clamped in the region of its edge between the upper and the lower tool whereby the shaping pressure is built up by decreasing the mutual distance of the two tools between which the edge of the rim is clamped. The workpiece, being in the form of a vehicle rim, rotates between the two pressed together rollers whereby the one roller acts on the outside and the other roller acts on the inside of the rim. The disadvantage of this device is the that the surface pressure on the outside is nearly three times greater than the corresponding surface pressure on the inside of the rim.
These facts can be seen from FIG. 1 showing a diagrammatic view of a known device for cold-roll forming of a vehicle rim. Reference numeral 1 designates the upper, roller-shaped tool, reference numeral 2 the lower tool having the shape of a roller too, and reference numeral 3 the workpiece. The tools 1 and 2 are pressed against the workpiece 3 with a force P. The specific pressure on the outside of the workpiece 3 is designated with P.sub.a and the specific pressure on the inside of the workpiece 3 is designated with P.sub.i. The corresponding radii of the rollers and the workpiece are designated with ra, ri and rw.
In order to calculate the specific pressure on the outside and on the inside, the formula of Herz is used: ##EQU1## Thereby, rm designates the means radius of the two roller bodies having the radii r1 and r2, L designates the length of the roller bodies and E designates the modulus of elasticity. Further, the following condition must be considered: ##EQU2## Using these equations, the person skilled in the art will come to the result that the following equation is true: ps EQU pa=pi.multidot.2.77.
This means that the specific pressure on the outside of the workpiece is 2.77-times greater than the specific pressure on the inside of the workpiece.
This disadvantage is accepted in the manufacturing of steel rims because steel rims can be shaped in three consecutive steps of cold-roll forming. However, in manufacturing annular bodies consisting of light alloy, e.g. of aluminium, the conditions are different. In this case, this disadvantage can not be accepted because the soft aluminium reacts to the uneven specific surface pressures very unfavourably.
Consequently, due to these reasons, the manufacturing of vehicle rims consisting of aluminium is particularly difficult and expensive. Usually, aluminium rims are manufactured as follows: Flat aluminium sheets having the required size are bent into the shape of tubes and the free edges are connected to each other by welding. Thereafter, the processing of the tubes is effected by means of tools in the shape of solids of rotation in numerous steps.
This manufacturing process comprises a number of important disadvantages. Firstly, the grain structure of the annular body of aluminium is changed at the edges where it is welded together; there, it gets an alloy like structure with the result that the strength is considerably reduced in the region of the weld seam. Tests have shown that particularly the fatigue strength is reduced to a fraction of the original strength with the result that a rim manufactured in the way as discussed hereinabove is susceptible to break which can have severe consequences.
Furthermore, the unequal surface pressure results in further great disadvantages. The soft aluminium, compared with steel, heavily reacts on unequal surface pressures so that external influences like shocks easily can lead to a deformation of the rim.